The present invention relates to the field of measuring and neutralizing electrostatic charge on moving dielectric materials. More particularly, the invention relates to real-time monitoring of charge density on moving material and the neutralizing efficiency of air ionizing devices in various manufacturing, converting and printing applications.
Surface charge on a continuous length of dielectric material can exist as a net or monopole charge and/or as dipoles of charge in isolated regions. Accumulation of such charge can occur in a wide number of circumstances and with a wide range of dielectric materials such as thin films, webs and threads made of paper, plastic, textiles, etc. Regardless of the form and/or material, however, the accumulation of net surface charge on a dielectric material presents potential electrostatic hazards that often need to be eliminated or significantly reduced. For example, reduction or elimination of net charge is important during operation in hazardous environments such as with an electrostatically-charged web moving in proximity to flammable vapors. Under such circumstances, web charge densities may increase sufficiently to spontaneously generate electrostatic discharges and ignite the flammable vapors.
Static charge on a moving dielectric material can be controlled in a conventional manner using ionized air molecules supplied to the material to neutralize the accumulated charge. For example, web charge is commonly reduced by an electrical, inductive or nuclear type of air ionizing device. To ensure the overall safety and effectiveness of the system, however, it is also necessary to monitor the efficiency of the charge neutralizing process. Conventionally, this is done by sensing the upstream charge density before the neutralization process and by sensing the downstream (or residual) charge density remaining on the surface after the neutralization process. This information can be used to calculate the ratio of the two charge densities that defines the efficiency of the charge neutralization process. Traditionally, such monitoring has been accomplished with dedicated electrostatic field sensors installed upstream and downstream of the neutralizer. Such conventional sensors are separate from, and in addition to the ionizers used to neutralize surface charge. Their use, therefore, introduces cost and complexity into conventional charge neutralization systems.
Most known electrostatic sensors of the type noted above are non-contact devices which are capable of measuring electrostatic field intensity or electrical potential created by a charged web. They are commonly referred to as field meters, electrometers or electrostatic voltmeters. Such devices may be mounted on web processing equipment in proximity to the moving web. In order to monitor web widths in the range of approximately 40xe2x80x3 to 80xe2x80x3, multi-sensor arrangements are are commonly employed to cover the width of the web. Alternatively, a segmented roller apparatus that operates in direct contact with a moving web may also serve as an electrostatic sensor for measuring charge density on moving webs.
Unfortunately, monitoring devices of the type noted immediately above are relatively expensive and require regular maintenance and calibration to ensure proper operation, especially in hazardous environments. Also, charge measurement with dedicated monitoring devices and charge neutralization with ionizers commonly take place at different physical locations along a web path. This inherently results in delayed ionization response times that vary depending upon the web velocity. This, in turn, may result in a high residual charge being left on the web, especially at higher web velocities, despite the fact that the system is being monitored for effectiveness.
It is also known in the art to measure ion current flowing through a single electrical neutralizer to a charged web by monitoring ground return current as described in U.S. Pat. No. 5,930,105 entitled xe2x80x9cMethod and Apparatus For Air Ionization.xe2x80x9d U.S. Pat. No. 5,930,105 issued on Jul. 27, 1999 and is hereby incorporated by reference. Monitoring return ground current as described in U.S. Pat. No. 5,930,105 offers the theoretical possibility that charge density upstream of the neutralizer, as well as the charge density downstream of the neutralizer can be monitored with the use of a single neutralizer. This is only possible, however, in an ideal case where charge neutralization is perfectly achieved over the lifespan of a neutralization system. As a practical matter, however, no such systems exist for a number of reasons. First, ionizer efficiency varies overtime due to deteriorization of ionizers through normal wear. Indeed, as ionizers approach the end of their useful lives, their ability to neutralize charge radically decreases. Further, users can also over-tax a neutralizing system by using it in a manner for which it was not intended. This could occur where, for example, the user attempts to neutralize the charge on a material that accumulates unusually high charge, or attempts to run the material at an unusually high velocity. Regardless of the cause, however, such factors all introduce a high level of uncertainty as to whether the intended charge neutralization has actually occurred in a given case. For this reason, conventional charge sensors are utilized in safety-critical applications.
In accordance with the present invention, static charges on a moving dielectric material are neutralized and the web charge density values before and after neutralization are determined from real-time monitoring of the ion current flowing from the charge neutralizing ionizers to the material. In particular, the present invention utilizes at least two charge-neutralizing ionizers which also act as charge sensors instead of employing dedicated sensors conventionally combined with dedicated ionizers. In this way, the effectiveness and/or efficiency of charge neutralization can be continuously monitored and the information obtained can be used to control the machinery which handles the dielectric material.
The present invention includes embodiments of a reliable, low-maintenance system with redundancy of charge neutralization and charge monitoring that includes a computer interface for displaying and/or storing information regarding various parameters such as charge density and the status of the charge neutralizers. In one apparatus embodiment of the present invention, a first ionizer responds to the charge density on a moving length of dielectric material to thereby reduce it. A second ionizer responds to any resultant charge which may have remained on the material and further neutralizes the resultant charge until little or no residual charge is left. A controller of the system responds to the sensed currents from the first and second ionizers, calculates various parameters such as the charge density on the moving material and generates control signals which can be used in a number of ways.